Club G40 Forum
Technical => Engine and Transmission => Topic started by: SteG40 on August 05, 2009, 06:52:52 pm
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Got the head off today and this is what I found. The Inlet valve's bent like I thought but there's damage to the head as well. I'll get a new set of inlet valves put in and the head fixed but there is marks and a nick out of the piston and i'm not sure what to do. Does it need repairing, is it scrap or can I leave it what do you reckon.
(http://img.photobucket.com/albums/v210/SteG40/Polo%20G40/PoloG40007-1.jpg)
(http://img.photobucket.com/albums/v210/SteG40/Polo%20G40/PoloG40014.jpg)
(http://img.photobucket.com/albums/v210/SteG40/Polo%20G40/PoloG40015.jpg)
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ooo nasty
just wondering why there are those marks on the head and piston.........what exactly happened? Looks like the cylinder has eaten something like some debris or something.
you'll be best getting a replacement piston as there seems to be quite alot of damage to the piston, i don't know if you can get single 1341cc pistons...........
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that's been caused by detonation! It's looks like that cylinder has got seriously hot
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Yep, pitting = det.
Standard pistons probably would look a lot worse if they'd been in. Holed piston results from severe detonation.
What was the compression on that cylinder after damage? -Just read the inlet valve was bent too, so no compression.
Cylider head can be repaired.
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The damage is from when I bent a valve. Would that cause det? I didn't hear any pinking or is it just a coincidence that it's the same cylinder and that there's something else wrong too?
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Detonation damage occurrs from explosive hammer action due to incorrect ignition timing. Will need repairing that piston because imperfection on the surface leads to a domino effect creating further damage.
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mmmmm seems very bad for det damage considering you didn't hear any pinking going on.
what are the colour of the plugs like? is cylinder 1 running hotter than the others going by the colour of the plugs?
i wouldn't bother repairing the head, just get a another one and transfer the exhaust valve.
i am not 100% convinced its det damage, i would have thought you would have melted a valve first.
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not sure that head will be reusable as there's a massive pit in it at the bottom of the pic. There's only so much you can skim off the head. haynes might have the minimum head thickness in it...
is this a ported head or standard ports?
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The damage is from when I bent a valve. Would that cause det? I didn't hear any pinking or is it just a coincidence that it's the same cylinder and that there's something else wrong too?
det is when tiny pockets of fuel ignite before it is meant to which is mostly due to high temps caused by wrong fueling, wrong ignition timing (too advanced), too high compression ratio, shit fuel etc.
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also, like robin said, what does the spark plug look like? have bits broken off it?
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It's a BVH i'm going too take the head to a local place and see what they reckon. Spark plugs are fine nothing broken off all the same colour too
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this is why i don't think its det without looking at the head phyically, with the plugs being all the same colour it suggests that cyl 1 wasn't running that hot.
Also for det to damage the engine that much i think you would most certainly hear the engine pinking its arse off, i had bad det on my lader set up and ran that for about 1k miles with it detting its arse off and the head and piston showed no damage whatsoever when i changed the head gasket.
i am still wondering if something has gone through your intake system, but having said that i have just look at the picture of the head and that big hole at the bottom looks like it has melted!!!!
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plus to me that piston looks like it's been fecking hot, looks like the end of a spark plug from an engine thats been running lean.
yep best bet is to take it to an expert, obviously anything we say here is just guess work without having the car here.
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mmmmmm looking at it in more detail it does look like det more than anything else.
get your injectors tested and cleaned.
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That large pit on the cylinder head matches the one on the piston so i'd guess its a melt. Did you clean all the coke off the top of the cyl1 piston crown before you took the photo?
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Yeah i cleaned it up abit it looked like the others before. I'll check the injectors but they were cleaned and rebuilt when i had the engine done. Also it happened on the dragstrip so I'm guessing there would be alot of heat in there.
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There's quite a lot of pitting that looks quite geometric/angular. I reckon that's secondary and probably a bit of plug or somesuch as suggested. That could be previous damage. The action seems to be at the other side of the head but if that's det. then something must have gone badly wrong very suddenly. Coke problems would show early with rough running long before the big bangsurely?
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Yeah i cleaned it up abit it looked like the others before
ah right, maybe it's just the fact you've cleaned it and then glare from the camera making it look white.
Let us know what they say when you get the head looked at.
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mmmm this is interesting seeing as you cleaned off the crap off the pistons.............
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steve from pitstop built this up didnt he? have you got a r32? i remember him telling me about one he built.
anyway its hard to tell cos you have cleaned it but if your injectors are ok then it could be due to a loose spark plug. this can cause det and over heating of the cylinder.
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I am still convinced something has gone through your intake system, it won't be det that has caused the damage as the indents have too many sharp edges, is the spark plug tip still on the spark plug?
Also check that your turbo hasn't chewed through anything which has then gone into cylinder one.
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I've never seen someone melt a piston and damage the head like that - so whatever the cause it's very serious! You'd have definitely heard and noticed detonation if it'd been the cause.
Is the coolant system okay?
What state are the valve guides in?
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i had something simmilar happen to my engine but no marks like that. mine melted No 4 piston, bent no4 exhaust valve, cracked the head across every port and into plug holes, and No3 piston fell to pieces when i took it out.
i would advise taking all pistons out and check them inbetween the top two rings cos a big chunk fell out of mine with no other damage to the piston.
if the engine is nearly std i have a brand new py piston and good SH injectors that i tested at work.
I blame my injectors cos after replacing them never had a problem.
If your pistons are 1341cc you might get a single one from Agra precision engineering in dundee 01382 229333. cost approx £200
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My brother took the head to work today he's a mechanic they thought that something has got in there too, i've checked the plugs and turbo again and everything looks fine so signs of any damage or bits missing.
Coolant systems fine everythings fairly new. Haven't checked the valve guides i'll get the place that's fixing it to check everything.
steve from pitstop built this up didnt he? have you got a r32? i remember him telling me about one he built.
Yep that's the one.
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Looking at the pics on a better monitor, the valves look in lots better condition that those from melted piston engines - mine were pitted slightly from the det when I melted one several years ago.
Does look like something's been mashed between the piston and head. Just need to find out what it is...
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your turbo wasnt water cooled was it?
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I have seen det marks before but it usually looks like some sort of errosion, your cylinder head looks like someones been at it with a hammer and chisel. Do you know what caused the valve to bend? I would suggest removing your piston and looking very carefully at the rings and lands. Det has a habit of breaking ring lands, but personally I wouldnt assume det from looking at the head. possible FOD or broken ring etc has got in there.
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Oh and with the head I would just lightly dress it up and re fit it, as long as there are no protrusions to cause hot spots then there shouldnt be much of a problem.
And study the head gasket for signs of errosion, a good burst of det will attack your head gasket as much as anything else.
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Do you know what caused the valve to bend?
Just had a thought I put the bent valve down to my bad launch on the 1/4mile and the revs going way too high but I wonder if something has got inside in the head that's whats actually bent the valve.
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to bend a valve you'll need to do way over 7000rpm to bend a valve!
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It hit nearly 8k for second or so, anyway engineering place has had a look today and both inlet and exhaust valves are bent they also reckon something has got in there. I'm guessing you can't by single exhaust valves for a big valve head so I'll have to get a set from jabbasport I think.
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jabbasport might sell them individually, you'll have to phone them I guess. I'm sure they used to list them individually on their old site before the recent refresh.
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depends what the big valves are from and who I suppose.
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jabbasport usually make the 31mm ones. That's what is in my head too.
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Valves shouldnt bend from over revving it, unless with all the revs and no load on your engine the piston has hit the valve. The valves should be well up to those revs. A lot of engines rev to well over 10k without particularly special valves. Plus over revving tends to cause valves to drop rather than bend.
Check the rods for length and the big end bolts for stretch.
Detonation usually occurs in long hard 4th gear up hill pulls etc rather than a quick launch. Especially in a polo where launching it puts relatively little stress on the engine due to a lack of traction.
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with hydraulic tappets and standard valve springs on these heads you are risking valve float much above 7k rpm.
Although I'd guess if something has got in there then it was big enough to get stuck between piston and valve and bend them.
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True, im not sure what effect excessively bouncing valves tends to have. I dont think they would bend though?
Im not sure how anything would get in there thats big enough to do that, I have heard of spark plugs disintegrating but im guessing that hasnt happened. Cam belt not jumped a tooth? I suppose there would probably be damage on other cylinders.
I have never had much to do with OHC hydraulic tappets/lifters, do they ever pump up and cause interference?
I would pull the piston/rod out for sure and check the ring lands/rings and the big end bearing/bolts. Then measure the deck height when its back together and compare with the other cylinders.
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Ste are you 100% sure the spark plug is still in one piece?
If a ring has dead and come into the cylinder then i would expect to see some scuffing around the bores of the cylinders.
something has gone through that intake system so its just a matter of trying to find what has come loose!
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Jabba only does the bigger exhaust valve, isnt your- inlet valve bigger--? inlet is the one on the left right? or on BVH's do they only make a bigger exhaust valve?
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Could a part have broken in the charger and made its way in there?
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Bingo look whats missing
(http://img.photobucket.com/albums/v210/SteG40/PoloG40020.jpg)
how do you get in there to put a new bolt in?
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pop rivot it
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What are those 4 bolts in the butterfly for?
If the butterfly bolts to the shaft then you can probably just unscrew it and take it out to fit the bolt, if it slides through the middle of the shaft then you probably cant. If you get what im saying.
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That explains it !
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ah ha! knew it wasn't det damage!
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What are those 4 bolts in the butterfly for?
If the butterfly bolts to the shaft then you can probably just unscrew it and take it out to fit the bolt, if it slides through the middle of the shaft then you probably cant. If you get what im saying.
it has to stay in the other bolts hold the wedge on the other side of butterfly this one has been dewedged and bolted to cover the holes but dont use nuts and bolts again i would say vibriations from engine will loosen them and you wouldnt want this sorta damage again great pics I hada similar problem on a 4 stroke bike and a grub screw stoped the engine reachin tdc and bent the valve etc too
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Nyloc nuts or loctite. A lesson for all of us there I think.
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pleased ya found what it was mate, who dewedged your throttle?
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Probably Beavis.
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least you found the issue. Iused nyloc nuts and loctite!
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Yeah PSD did it a few years ago.
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I used liquid metal on mine, put a chamfer on both sides of the holes first and then when the resin dries it's an hour-glass shape so should stay in place. Also if it was to come out it'd be a lot smaller than a nut & bolt so less chance of damage occurring.
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Just checked mine - it's a standard nut and bolt. No sign of any thread lock on it either. Everyone should check theirs unless they did it themselves just in case!
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Just did mine, however the old bolts were threadlocked. Worth doing just in case :)
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Just a quick update on this finally got it all back together yesterday and it still won't start it's the same as before.
It turns over and tries to start but just misfires, I've done a compression test and cyl 1 with the damage is just over 100psi and the others are all around 140psi. It looks like the damage to the piston is too much. I think i'll pull it apart, have a look whats up and rebuild it but is it worth chucking more money at it?
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hmm, that's not good to hear. I'm surprised it wont run though, the old engine I had in mine was down on compression on cylinder 3, it was less than 90psi and it still ran ok, just breathed really heavily. I even did a track day with it like that and it ran ok, it used oil but was ok.
Does the misfire stay the same or get worse if you remove the plug lead from that cylinder? Are you sure the valves are sealing properly? Might be worth doing a leakdown test.
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Have the valves been re-seated ?
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Just joined the forum & thought i would have a read of some isues, having read most of these posts it's great reading different opinions mine is that when the engine got hungry & ate a bolt it broke the top of the piston thus gripping the top compression ring to tightly hence the loss of compression on this cyl...
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hats off for robin for pretty much diagnosing it from the start!